Speaking at the 40th anniversary of the Rape Treatment Centre, the 76-year-old told the audience that her mother, Frances Ford Seymour, had been sexually abused before ending her own life when Jane was just 12-years-old.
The Hollywood star said she discovered that her mother had been sexually abused after reviewing her medical records while writing her memoirs.
"The minute that I read that, everything fell into place," Fonda said. "I knew why the promiscuity, the endless plastic surgery, the guilt, the inability to love or be intimate, and I was able to forgive her and forgive myself."
The Rape Treatment Centre provides free treatment for sexual assault victims.
Last year, Jane spoke candidly with The Weekly about her adolescence, which she described as "challenging".
"I come from a family of depressed people, a long line of depressed people on both sides," she told The Weekly.
Yet, the actress said she was happier than ever.
"I can honestly say I have never been happier. Part of getting older is knowing what you don't need in life," she said in the interview. "You can't let things go and become lighter. And it's wonderful to finally feel this way."